If SMRs are cheap enough to act as backup to wind and solar, they are cheap enough to displace wind and solar entirely. And the contrapositive as well: if SMRs are not cheap enough to displace solar and wind, they aren't cheap enough to act as backup. The scenario where it's just a backup never arises in cost minimized solutions.

> If SMRs are cheap enough to act as backup to wind and solar, they are cheap enough to displace wind and solar entirely.

That doesn't follow necessarily. Wind & solar being the most cost effective doesn't mean you remove all backups just because they aren't as cost effective.